The heart can be thought of as an electro-biomechanical pump. Electrical signals stimulate muscles to contract in a controlled manner to achieve complex pumping actions. It is known that aberrant conduction of electrical activity in the heart can lead to arrhythmias that may cause morbidity and/or death.
Drugs have been developed to treat these conditions but they have low efficacy in many cases and dangerous potential side-effects. An alternate technique is radio-frequency ablation of the sites in the myocardium (heart muscle) believed to be responsible for the initiation or maintenance of arrhythmias.
During normal heart operation there are multiple electrical signals that travel through the myocardium triggering a muscle response. It can occur that an anatomical region changes its conductive properties (perhaps due to injury) thus causing the electrical signals to be diverted or blocked. In some cases an anatomical obstacle or functional alteration in myocardial electrical properties can cause the electrical impulse to degenerate into wavelets that circulate and suppress the normal activation signal. Changes in conductive properties may cause a signal to arrive sooner, or later at a site remote from the source of the abnormal circulating activity. In either case the normal sequence of activation of contraction of heart tissue is disrupted.
Radio-frequency ablation seeks to treat the arrhythmia by ablating the source of aberrant signals or interrupting circulating electrical activities at a critical pathway. To do this it is essential that the critical pathway or abnormal signal source be accurately located. This is done using endocardial mapping techniques.
The most basic form of mapping of the electrical activity in the heart is done by moving a catheter with electrodes within the chamber of the heart and observing the resultant electrical signals (electrograms). The clinician compares a recording made at one position with recordings made at other positions. Example parameters extracted are: the relative timing of electrical activations or the shape (morphology) of the electrical signal. Abnormal behaviour can be localised in this way.
The measurement of relative time delay across the endocardial surface is facilitated by using more than one catheter device. For example if one catheter is kept in a fixed position and another is moved around, then the clinician can determine the relative electrical timing relationships from one location with respect to another.
An extension of this approach is to use catheters with multiple electrodes. This allows electrograms at several positions to be measured at the same time.
A further extension to this is called global mapping of the heart chamber activity. Electrograms are recorded simultaneously over the whole chamber using the appropriate multi-channel device. A fundamental limitation here is the size of device that may be inserted into a vein to encapsulate the wires required for connection to each electrode. With global mapping the timing relationships between various locations can be viewed simultaneously. To some extent this takes a mental load off the clinician who is required to remember the various timing relationships.
Medical device technology has emerged in the last few years making possible the capture of signals from many electrodes simultaneously. An example is a device such as the so called basket catheter. This appears to have been first described in U.S. Pat. No. 5,156,151 titled “Endocardial Mapping and Ablation System and Catheter Probe”, assigned to Cardiac Pathways Corporation, and related patents.
A limitation with this sort of mapping device is that the electrodes are generally spaced too far apart to resolve fine spatial detail. Another problem is maintaining contact of the electrodes with the heart chamber wall. The number of electrodes is limited by the diameter of catheter that can be safely inserted. Individual wires are required to connect to each electrode. There are typically 64 electrodes.
The concept of global mapping lends itself to the display of isochronal maps and velocity maps of the endocardial surface. An isochronal, or equal time map, is a graphical representation of locations on the endocardial surface where electrical activation times are the same. The lines or contours that join these locations of equal activation time represent wavefronts of electrical activation. The electrical wavefront can be thought to travel in a direction perpendicular to these contour lines. Contour lines are generally shown at equal increments in time or alternatively colour is used to indicate equal time increments. When contour lines are spaced close together this indicates a lower velocity (ie it takes a wavefront longer to travel a certain distance). This graphical representation using a contour map can be referenced to the anatomical location of the measurement positions. Maps can be represented in a 2D or 3D fashion.
The velocity of wavefront propagation can be computed from the measured time delay between known electrode locations. In a similar fashion to the isochronal map a graphical display can be used to represent the variation of this parameter on the endocardial surface.
These mapping concepts have been used by researchers for many years.
Such a system is described in U.S. Pat. No. 5,487,391, titled “Systems and Methods for deriving and displaying the propagation velocities of electrical events in the heart”, assigned to EP Technologies. In this patent the activation time is measured at locations of an array of spaced apart electrodes (eg a basket). An algorithm is described to compute the spatial gradient of the electrogram activation time. This inverse of the magnitude of this value is taken as the velocity of propagation. A colour display is described to represent different velocity magnitudes at their relative spatial locations. The patent also refers to the display of the magnitude of other physiological parameters measured by an array of spaced apart sensors. The invention basically provides a snapshot and does not allow parameters to be observed over time.
EP Technologies describe a related system in their U.S. Pat. No. 5,494,042, titled “Systems and Methods for deriving electrical characteristics of cardiac tissue for output in iso-characteristic displays”. This patent describes means for deriving an electrical characteristic of tissue lying in multiple paths between spaced apart electrodes. The electrical characteristic in this particular case is the tissue impedance. The patent also describes means for creating an output displaying in three dimensions, groups of equal electrical characteristics in spatial relation to location of the electrodes on the structure. Parameter values are assigned to an interpolated mesh. Colours are assigned to these values.
It is clear that useful information is obtained by the appropriate graphical display of physiological parameters spatially referenced to the endocardial surface of the heart.
Another technique for global mapping is a non-contact mapping technique. This is implemented in a commercial system known as Ensite by Endocardial Solutions Inc. This system is originally described in two patents assigned to Endocardial Therapeutics Inc: “Endocardial Mapping System” (U.S. Pat. No. 5,297,549) and “Heart Mapping Catheter” (U.S. Pat. No. 5,311,866).
U.S. Pat. No. 5,297,549 describes the limitations of traditional direct contact electrodes. These include spatial averaging effects due to the area of the electrode. The patent also describes the limitations of electrical potential map creation as a result of interpolation based on a “limited set of measurements”. The Ensite system takes measurements made from a high resolution electrode array catheter device located inside the heart chamber, not in contact with the wall, and using mathematical extrapolation techniques, produces maps of the electrical potential at the endocardial chamber wall. The method relies on locating the endocardial surface accurately. An improvement to the location method is described in: “Endocardial Measurement Method” (U.S. Pat. No. 5,553,611). This uses an additional pair of excitation electrodes to generate an electric field inside the heart. Distance to the heart wall is derived from impedance measurements.
While in theory a high spatial resolution can be obtained, in practice the accuracy of the extrapolation of the electrical field is limited.
An advantage of the Ensite method (as described in the summary of the patent) is that an activation map can be created from a single heart beat once the geometry has been created. The maps created can be followed over time. This is unlike the roving catheter technique. Global mapping has an advantage over sequential mapping when there are unstable patterns of activation occurring ie activation patterns that may change significantly from beat to beat. This change may only be in a particular spatial region. The roving catheter technique relies on the spatial activation pattern remaining the same within the time of the roving process.
However even with the availability of multi-channel mapping devices clinicians in practice still rely on manually manipulating a flexible catheter around the heart chamber to localize electrical activity to very specific regions.
To enhance this approach methods have been devised to spatially locate the roving electrode within the heart chamber in 3D. Acoustic means, magnetic field means and electric field means have been used for this purpose. A map is built up by moving the roving electrode around keeping track of its location in 3D space and measuring the electrogram sensed at each location. Such a system is described in U.S. Pat. No. 5,391,199, assigned to Biosense Inc. This patent describes a 3D catheter position location system. The location system generates position signals in the catheter tip in response to externally applied magnetic fields, generated by electromagnetic field generator coils situated outside the body.
The method allows a 3D representation of activation timing to be generated. An advantage of this approach is that it obviates the need for continuous fluoroscopic imaging to locate catheters (which is not desirable because of X-ray exposure to patient and operators). The fluoroscopic imaging approach has limitations anyway because it provides only a 2D cross sectional view (a 3D perspective may be obtained by sequentially rotating this view).
A 3D graphical display may be built up by moving a single catheter around the heart chamber. This technique is used in the Carto System available from Biosense Inc.
Another possible method of catheter localisation is by using acoustic signal means. The Real Time Position Management System by Cardiac Pathways is such a system. This is described in U.S. Pat. No. 6,216,027, assigned to Cardiac Pathways Corporation. This system uses one or more ultrasound reference catheters to establish a fixed 3D coordinate system within a patient's heart using triangulation. The coordinate system is represented graphically in 3D on a video monitor.
Another 3D localisation method is called LocaLisa from Medtronic Inc. This system senses impedance changes between a catheter and reference points.
In the previous two methods reference electrodes are kept in fixed positions which are located in known regions and the position of the roving electrode is tracked relative to these fixed positions.
Despite the developments in electrode design and the improvement in signal measurement, the clinician still has a limited range of data from which to assess the correct location for tissue ablation. Additional tools for visualisation and assessment are required.